1. Field of the Invention
The present invention relates to a diamond covered member useful for semiconductor substrates, heat sinks, various protection films, optical parts, and the like. The present invention also relates to a process for producing the above diamond covered member.
2. Background Art
Diamond crystal, which has high hardness, high thermal conductivity, and excellent chemical stability, is widely used for cutting tools, abrasion-resistant tools, heat sinks (heat-releasing parts), and so forth. Furthermore, diamond semiconductors, which are doped with an impurity such as Group 3b and 5b elements of Periodic Table, have many advantages such as the band gap (5.4 eV), thereof being much larger than that of silicon semiconductor, the higher mobility of the positive holes therein, the mobility of electrons therein approximately equal to that of silicon, the small dielectric constant, the high thermal conductivity, and so forth.
Thin diamond crystal films are generally grown in a form of a rough polycrystal film on a substrate made of silicon, molybdenum, or the like. In recent years, methods are disclosed for epitaxial growth of diamond crystal on varieties of substrates and for formation of diamond crystal having a smooth surface for the purpose of utilizing the above characteristics of diamond crystal.
For example, diamond crystal is known to grow epitaxially on a diamond single-crystal substrate by vapor-phase synthesis. Japanese Patent Application Laid-Open No. 63-252998 discloses growth of a diamond single-crystal on a silicon carbide single-crystal substrate. Japanese Patent Application Laid-Open No. 64-3098 discloses growth of a diamond single-crystal on an oxide single-crystal substrate. Japanese Patent Application Laid-Open No. 2-233591 describes that a single-crystal diamond layer is formed when the lattice constant difference between the diamond crystal and the substrate is less than 20% and the molar ratio of carbon atoms and hydrogen atoms is in the range of from 1 to 10%. Japanese Patent Application Laid-Open No. 1-301864 describes the formation of thin film having maximum surface roughness of 200 .ANG. or less composed of a mixture of diamond crystals, graphite crystals and amorphous carbon under the formation conditions of an organic gas concentration of 3 mol % or higher, and use thereof for a forming die for glass.
It is known that a diamond single-crystal substrate is extremely expensive, and is not useful practically. The epitaxial growth on a silicon carbide single-crystal substrate or an oxide single-crystal substrate gives granular crystals as the results of epitaxial growth of granular diamond single-crystals in a dispersed state, so that the formed film is poor in surface smoothness, usually having surface roughness (Rmax) of several hundred nm or more. Moreover, such a single-crystal substrate is extremely expensive generally and is disadvantageous in cost. The epitaxial growth on nickel, nickel-copper alloy, copper, or the like described in Japanese Patent Application Laid-Open No. 2-233591 gives similarly grown granular diamond crystals, and the resulting film is also poor in surface smoothness disadvantageously.
The thin film composed of a mixture of diamond crystals, graphite crystals, and amorphous carbon which is described in Japanese Patent Application Laid-Open No. 1-301864 gives a diamond thin film having maximum surface roughness of 200 .ANG. or less. However, the incorporation of graphite in the crystal film deteriorates chemical stability, oxidation resistance, and thermal conductivity of the film, rendering the film practically useless for a semiconductor.
A specific plate-shaped diamond was proposed by the inventor in EPC Patent Application No. 93104860.7. The inventor's proposed plate-shaped diamond is formed on a substrate by vapor phase synthesis, and is characterized by the ratio of the length of a diamond crystal in a direction perpendicular to the substrate face to the length thereof in direction parallel to the face of the substrate ranging from 1:4 to 1:1000, and an upper face of the diamond crystal making an angle of from 0.degree. to 10.degree. to the face of the substrate. A method for forming the plate-shaped diamond crystal is also disclosed in the same EPC Patent Application, and is shown to be useful for electronics materials. The EPC Patent Application, however, does not describe growth of a plurality of plate-shaped diamond on a substrate and coalescence thereof into a film, nor uses of the diamond film.